1. Field of the Invention
This invention relates to a method of scanning a specifying magnetic field for imaging and measuring internal information of a body being measured (to be referred to as a target hereinafter) by using nuclear magnetic resonance (to be referred to as NMR). More particularly, the invention relates to a method of scanning a specifiying magnetic field, which is generated by applying a local static magnetic field different from the surrounding field to an area being measured, so as to facilitate extraction of information only from the area being measured by using the difference in the resonance frequency, for instance the NMR information relating to nuclear magnetic substances in the target, such as hydrogen (H),fluorine (F), sodium (Na), potassium (K), magnesium (Mg), carbon (C), and phosphorus (P) in the inside of the target, and the invention aims at provision of a scanning method which facilitates production of a practical electric scanner of light weight and simple structure for scanning the specifying magnetic field produced with a comparatively low electric power.
2. Description of the Prior Art
In typical conventional methods of scanning the specifying magnetic field for NMR imaging, such as the method disclosed by the inventors in their Japanese Patent Laid-Open Publication No. 133,192/1979 relating to a method of producing specifying magnetic field, a line-shaped focusing magnetic field is generated by using a group of parallel wires so as to produce line-shaped specifying magnetic field in a line-shaped area within a target for NMR measurement, and the thus produced specifying field is scanned by regulating or switching the distribution of electric currents in the group of parallel wires. The conventional methods of scanning the specifying magnetic field have a shortcoming in that the coils for carrying electric currents are complicated, that a switching circuit is necessary for regulating the distribution of electric currents, and that the driving of such switching circuit is complicated.
As to the configuration of the specifying magnetic field .DELTA.H.sub.s to be scanned by the conventional methods, there are number of variations; such as the line-shaped focusing magnetic field having magnetic field equipotential surfaces of cylindrical shape extending in the direction of the static magnetic field H.sub.o in the measuring area, the spherical or ellipsoidal focusing magnetic field having closed equipotential surfaces of spherical or ellipsoidal shape, and the focusing magnetic field having equipotential surfaces which open to the surrounding in a trumpet- or drum-like shape. With the conventional methods, it has been noted that complicated processes are necessary to scan the specifying magnetic fields of such wide variety.
Further, in the conventional methods of producing the specifying magnetic fields such as the inventors' method disclosed in the above-mentioned Japanese Patent Laid-Open Publication, the line-shaped specifying magnetic field or a line-shaped focusing magnetic field is produced in the target by using a group of parallel wires so as to extract the NMR information from a cylindrical portion of the target, and such line-shaped specifying magnetic field is formed by synthesis of a long homogeneous static magnetic field extending in a longitudinal direction and a magnetic field with no components in the longitudinal direction produced by the group of parallel wires. Accordingly, the group of parallel wires are very long as compared with the spacing therebetween, and the coils for producing the line-shaped focusing magnetic field are much bigger than coils for detecting the NMR signals. Consequently, the conventional methods have shortcomings in that a driving system with high power is necessary to produce the required specifying magnetic field and that a large amount of coil material is necessary so that the NMR device becomes heavy.
Regarding the details of the conventional methods of producing the above-mentioned specifying magnetic field, the line-shaped specifying magnetic field has closed equipotential contours on a plane perpendicular to the static magnetic field, the closed equipotential contours having no electric current therein and being parallel to each other in the direction of the static magnetic field in a certain area, and such line-shaped specifying magnetic field is produced by disposing a group of wires substantially in parallel to the direction of the static magnetic field and applying such electric currents to the group of wires that the magnetic fields induced by the group of wires cancel each other at a certain spatial point. Theoretically, it is possible to produce such line-shaped specifying magnetic field by synthesizing a homogeneous static magnetic field and a magnetic field having components only in directions perpendicular to the direction of the homogeneous static magnetic field, said magnetic field components being zero on a certain spatial line while being non-zero in other areas. p In view of such theoretical possibility, if a group of differential coils are formed by a plurality of closed loop coils and electric currents of such magnitudes and such directions are applied to the differential coils that magnetic fields induced by the closed loop coils are cancelled by each other at a certain spatial point, and if a synthesis is tried by superposing that static magnetic field which is homogeneous in all directions except at the aforesaid certain point onto the magnetic field induced by the closed loop coils, such synthesis has a shortcoming in that the variation of the intensity of the synthesized magnetic field is predominantly influenced by the superposed static magnetic field and the synthesized magnetic field is nothing but a gradient magnetic field having a gradient of magnetic field intensity in the direction of the superposed static magnetic field.
However, in the above-mentioned synthesis of the magnetic field induced by the group of differential coils and the static magnetic field, if there should be a certain means for cancelling that component of the synthesized magnetic field which is in the direction of the superposed static magnetic field in a selected area, the magnetic field component in the direction perpendicular to the direction of the superposed static magnetic field becomes predominant in the synthesized magnetic field, whereby the line-shaped specifying magnetic field or the line-shaped focusing magnetic field can be obtained.